Hitherto, zinc-rich coatings, serving as anticorrosive undercoatings for a heavy-duty coating system, are generally categorized into solvent-based inorganic coatings containing an alkyl silicate binder (see, for example, Patent Document 1) and solvent-based organic coatings containing a binder of high polymer resin (e.g., epoxy resin or polystyrene resin) (see, for example, Patent Documents 2 and 3).
At present, reduction in VOC level and saving petroleum resources are global environmental issues which are under discussion on the worldwide scale. Among them, reduction in the amount of organic solvent, which is produced from petroleum and is a source of VOC, is the most critical problem to be solved in the coating/painting industry. According to a guideline of Japan Paint Manufacturers Association, the amount of organic solvent employed in generally used solvent-based coatings should be reduced to 50% of the amount of organic solvent actually used in 2004.
The coating industry has taken the issue seriously and has proceeded with conversion of organic-solvent-based coatings to non-organic-solvent-based coatings such as aqueous coatings and powder coatings, so long as the conversion is technically feasible. In the fields of building construction, automobiles, light electric appliances, general industrial products, etc., most of the coatings used therein have already been converted to those of a non-organic-solvent type.
Generally, such zinc-rich coatings are painted in combination with epoxy undercoating, non-yellowing urethane overcoating, acrylic silicone overcoating, fluorine-containing overcoating, etc. Hitherto, techniques for producing these coatings from aqueous resin have been virtually established, and these coatings exhibit satisfactory performance of a practically employable level. In contrast, for zinc-rich coatings, zinc shop coatings, and the like, no aqueous vehicle providing the coatings with long-term durability has been developed. Thus, conversion of organic-base coatings to aqueous coatings has not been fully accomplished in the fields where heavy-duty zinc-rich coating is required (e.g., bridges, oil tanks, ships, and structures in harbor area).
Needless to say, the performance most required of a zinc-rich coating is provision of a long-term anticorrosive property to steel material. Among aqueous coatings, technically, aqueous organic coatings are more readily produced, and some aqueous coatings for industrial uses have been employed in practice. However, currently employed aqueous organic zinc-rich coatings are unsatisfactory in terms of anticorrosive performance, and great difficult is encountered in attaining the performance of a zinc-rich coating required for a heavy-duty coating system. Thus, there is demand for an aqueous inorganic coating that is envisaged to exhibit a longer-term anticorrosive performance.
Conventionally, alkali silicate salts and colloidal silica have been studied as typical materials for providing a binder that can be used in an aqueous inorganic coating, and a number of patent applications thereon have been filed. However, even though a number of inventions related thereto have already been patented, unfortunately, no zinc-rich coating that can be used in practice while meeting the demand for long-term anticorrosive coating systems against environmental problems is currently supplied on the market. The reason for this is that the performance of conventionally studied aqueous inorganic zinc-rich coatings has not yet attained the performance level required by the heavy-duty coating system.
When only an alkali silicate salt is employed as a binder, the following problems occur. Specifically, 1: when the alkalinity of the alkali silicate salt is excessively high, reaction between the alkali silicate and zinc powder unavoidably occurs. In this case, the pot life is as short as about 1 to about 3 hours, making handling of the coating difficult. 2: Unevenness in color of the dry coating film and surface blushing of the coating film after exposure to the atmosphere may be problematic. 3: Although the protective potential of zinc is ensured, reaching the potential may require a long period of time. 4: Although the coating exhibits excellent anticorrosive performance in the salt spray test, the stability of the coating is poor. 5: The adhesion of the coating to an epoxy undercoating is variable. 6: When immersed in city water, dissolution, blistering, and peeling of the coating readily occur. The coating cannot be used in a process involving immersion.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2004-359800    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. Hei 11-124520    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2005-66574